Its a bit vague to answer properly as what happens would I imagine depend heavily on the circumstances but 9 times out of 10 I can't see you losing it unless you've done something illegal. If only because anyone born in a corp is probably in possesion of a full SIN and even if they arent that's a huge investment not just in education/training but also loyalty given the shadowrun world they've been raised from childhood to believe the corporation is their family and they'll have attended the Sony sporting arena, shopped at the Sony supermarkets, attended the Sony schools all inside the Sony arcology. So while the corporation may not actually care about them individually they wouldn't just casually toss all that aside. A limited SIN wont necessarily have that but they'll be paying higher taxes as well and taking that off them is going to lose you that income. Its important to remember most corporations able to issue a SIN aren't going to be Tom's Plumbing servicing a small town they're going to have multiple interests with both multiple jobs for a particular field and multiples fields of work (agriculture, manufacturing, mining, genetics, RnD, etc) so even if you're fired from a specific job that doesn't mean you're fired from the corporation.
Some examples of how it could go as I see it . . .
1) Fired via redundancy because their job simply isn't needed anymore or at least not to be done by a living being: No reason to take away the corporate SIN since your still a loyal productive employee of the corporation its just you'll need to find another job to work in. Probably with some corporate redeployment "Sorry but we no longer need a trained electrician to monitor this emergency shut down button however we have many lucrative jobs in the electrical field please review the following listing and click the interest button to submit your resume." Its probably even automated matching your skills to the current openings and may require you to relocate to another country if you want to keep working and eating and anything else involving money. However just because the corporation doesn't care doesn't mean it will want to lose a corporate citizens skills/revenue.
2) Fired via taking the fall for your bosses mistakes (an example in one of the source books of where fixers come from in Japan): You'd retain the corporate SIN and be "Fired for your crimes" before the boss thanks you gratefully and introduces you to the off the books positions as a facilitator. Sure you took the blame but for everyone who matters they know you didn't do it and there'll be plenty of nice perks coming your way for your loyalty.
3) Fired for incompetance: Again you'd keep the SIN or at worst be downgraded to the limited variety because while you mucked up that's no reason to deny the corporation any future taxes from you.
4) Fired for embezzelment/crimes of some nature: Definately a loss of corporate SIN but you get a shiny new criminal one to replace it.
5) Fired for treason i.e. being the inside man on a paydata extraction: A permanent reassignment to their underground toxic waste dump examination facility currently located six feet under the nuclear disaster site of your choice.
I honestly can't see someone with a corporate SIN (limited or full) even be booted to the curb as you put it since there's not a lot of situations where that would be the outcome. It'd have to be something like a long history of poor performance in lots of different jobs ranging all the way down to basic janitorial or manual labour. Something criminal is going to usually see criminal charges fines, jail time, etc and anything severe enough to be fired from your job would probably be severe enough to have an associated jail time/criminal SIN attached. General dismissal with good performance well it wouldn't cost the company much to make a search engine that match's the employee's extensive records to the positions available across the corporation and that way your not so much firing them as reassigning them while retaining their taxes/loyalty/families loyalty etc.
Of course to me since a corporate SIN isn't really like a corporate ID today but like a national citizenship getting one given/taken away wouldn't happen easily especially a full one you'd have to really transgress just like those hired onto the corporation get a limited SIN not a full one. However its the full one's your talking about here with born and raised in the corporate fold and that's the difference between being a citizen of country X and having a visa to work in country X. I could see situations where the limited SIN is taken away especially with a new hire who doesn't perform up to the corporate standards but even those would be rare because with that group you would do a full background check before hiring them in the first place. The main groups being hired and tossed to the curb on firing would be the national SINNERs hired in job lots for basic jobs so when the Anrief factory closes down they get fired while those with corp SINs are reassigned to a similar job in either a still operating factory or another field. For example a security officer at the corporate headquarters who simply isn't suitable for that environment (I'm sure you can come up for a reason) is reassigned to a regional office then a facility being shut down then a small farming community with a population of a few hundred who all know each other and the biggest crime they have to deal with is childhood bullying or they are moved to a training job or files and records downstairs by the boiler or any of a dozen other jobs. The corp still gets their taxes, they're still employed, a job is filled everyone wins, more importantly wins (maintained loyalty, maintained taxes, jobs filled especially unpopular ones such as the resident electrician on an artic research base).
Which doesn't even consider the era this all split aside and there may still be a strong Japanocorp ethos influencing decisions with people getting promotions/pay raises based on seniority rather than talent, people reassigned to a office that isn't actually doing any work (like the simpsons had for teachers in one old episode). Then there's the old glitch in the system like what happened in office space with someone being fired but it not actually registing in the system so they kept getting paid . A local business here had a foundry they largely shut down but didn't want to fully decomission in case it was useful again in the future so for years they kept on two staff a janitor to clean the place and a supervisor to ensure the janitor did his job by the time they finally shut it down fully the two guy's had turned in their retirement papers after years of essentially being paid a full wage to show up, play chess with each other and clean up after themselves.
Its the whole level of employement issue national SINNERs would be hired and fired every day, limited corp citizens would be carefully selected for valuable skills that would see them transferred elsewhere or a contract would be renweed, a corp citizen is practically corp property and its much better to place that somewhere else its useful than to just toss it in the garbage.